As is well known, one of the methods of increasing the power of an engine is to increase the engine speed or the number of revolutions of the engine. Several engines for two-wheeled vehicles, which are required to be of small size and to have high power, are known to have a practical maximum engine speed of more than 10,000 revolutions per minute. Since thermal efficiency is reduced when the engine speed is increased, it is necessary to control the reduction of thermal efficiency. For this purpose, it has heretofore been attempted to increase the number of intake and exhaust valves per cylinder. Four valve engines having two valves for intake and two valves for exhaust per cylinder are presently in practical use.
However, as the number of valves of a cylinder is increased, the opening and closing mechanism of the valve becomes more complicated. This gives rise to an undesirable increase in mechanical friction loss. This increase in mechanical friction loss will lead to a reduction in shaft horsepower, which is particularly noticeable at low load running, such as idle running.